The current trend in nappy construction is to produce even thinner constructions with reduced cellulose fibre content and increased superabsorbent content. The advantage of thinner constructions is exhibited not just in improved wear comfort but also in reduced costs in packaging and storage. With the trend toward ever thinner nappy constructions, the profile of requirements on the superabsorbents has changed significantly. Of crucial significance is now the ability of the hydrogel to conduct and distribute the liquid. Owing to the higher loading of the hygiene article (amount of superabsorbent per unit area), the polymer in the swollen state must not form a barrier layer for subsequent liquid (gel blocking). If the product has good transport properties, optimal exploitation of the overall hygiene article can be ensured.
In addition to the permeability of the superabsorbents (reported in the form of the “Saline Flow Conductivity—SFC”) and the absorption capacity under compressive stress, the absorption rate of the superabsorbent particles in particular (reported in amount of liquid absorbed per gram of superabsorbent per second) is also a crucial criterion which enables statements about whether an absorbent core which comprises this superabsorbent in a large concentration and has only a low fluff content is capable, on its first contact with liquids, of absorbing them rapidly (“acquisition”). In the case of absorbent cores with a high superabsorbent content, this “acquisition” depends, among other factors, on the absorption rate of the superabsorbent material.
From the prior art, there are various known property rights which are supposed to enable an increase in the absorption. WO 96/17884A1 discloses a water-absorbing resin for which a solid blowing agent with a particle diameter of 1 to 100 μm is used in the monomer solution. In principle, preference is given to organic azo compounds and here specifically to the acrylic salts of azo compounds containing an amino group. Pure carbonates, ammonium nitride or mixtures thereof can optionally be used.
Disadvantages here are the rapid conversion of the azo compounds and the basic dispersion of the small solid particles in the monomer solution. Larger particles cannot be dispersed without separation of the different particles in the dispersion when left to stand. U.S. Pat. No. 5,118,719 discloses the addition of carbonate blowing agents in the form of carbonates, bicarbonates or mixtures thereof, or the introduction of gaseous carbon dioxide into the monomer solution. The blowing agent may be present in the dispersion in solid or dissolved form and is added shortly before or after the induction of the polymerization. This increases the absorption properties, such as a higher swell rate and gel stability. A disadvantage here is that the blowing agent has already escaped from the superabsorbent hydrogel before or during the gel formation, or forms larger bubbles which do not ensure a microporous structure of the superabsorbent polymer.
The general use of blowing agents is likewise described in publications EP 664207, U.S. Pat. Nos. 5,712,316, 5,399,591, 5,462,972. This likewise involves the use of usually inorganic blowing agents which release carbon dioxide in suspension, or gaseous or solid carbon dioxide which are added to the monomer solution. Predominantly potassium and ammonium compounds are used. An increase in the absorption and the absorption under pressure has been observed.
In general, it is an object of the present invention to overcome the disadvantages arising from the prior art.
More particularly, it is an object of the present invention to provide a process for producing a water-absorbing polymer that has an improved swell rate and faster absorption of liquids, while simultaneously maintaining the overall quality. This process should also be performable in a simple manner and not need the use of organic additives.
It is a particular object of the present invention to provide a process by which water-absorbing polymers can be produced, and a particularly high swell rate can be ensured.
It is a further object of the invention principally to specify a water-absorbing polymer, composites comprising such water-absorbing polymers and chemical products comprising such water-absorbing polymers or composites, said water-absorbing polymers having an increased absorption rate for aqueous solutions.
It is a further object of the present invention to provide a process for producing water-absorbing polymers, ensuring homogeneous distribution and very homogeneous action of the blowing agents in the reaction space.
These objects are achieved by the subject-matter of the category-forming claims. Advantageous configurations and developments which can occur individually or in combination form the subject-matter of the dependent claims in each case.